IME composition events are handled inconsistently in WebKit

According to W3C DOM Level 3 events,
1) A browser should fire compositionstart, compositionupdate, and compositionend event during a composition.
2) The textEvent event should be dispatched after a compositionend event if the composition has not been canceled.
3) While a composition session is active, keyboard events should not be dispatched to the DOM (i.e., the text composition system "swallows" the keyboard events), and only compositionupdate events may be dispatched to indicate the composition process.

However, all WebKit ports handle composition & textEvent events in inconsistent ways. Even keyboard events are still dispatched to the DOM during a composition.

Therefore, it is necessary to fix the problems as follows:
1) IME Composition events should be handled consistently in all WebKit ports.
2) Keyboard events should not be dispatched during a composition.
3) The textInput event should be dispatched after a compositionend event.